Implantable medical devices (IMDs) are devices designed to be implanted into a patient. Some examples of these devices include cardiac function management (CFM) devices such as implantable pacemakers, implantable cardioverter defibrillators (ICDs), cardiac resynchronization devices, and devices that include a combination of such capabilities. The devices are typically used to treat patients using electrical or other therapy or to aid a physician or caregiver in patient diagnosis through internal monitoring of a patient's condition. The devices may include one or more electrodes in communication with one or more sense amplifiers to monitor electrical heart activity within a patient, and often include one or more sensors to monitor one or more other internal patient parameters. Other examples of implantable medical devices include implantable diagnostic devices, implantable drug delivery systems, or implantable devices with neural stimulation capability.
Additionally, some IMDs detect events by monitoring electrical heart activity signals. In CFM devices, these events include heart chamber expansions or contractions. By monitoring cardiac signals indicative of expansions or contractions, IMDs are able to detect tachyarrhythmia. IMDs are further able to provide therapy for tachyarrhythmia, such as high energy shock therapy or anti-tachycardia pacing (ATP). Tachyarrhythmia includes abnormally rapid heart rate, or tachycardia, including ventricular tachycardia (VT), supraventricular tachycardia (SVT), and sinus tachycardia (ST). Tachyarrhythmia also includes rapid and irregular heart rate, or fibrillation, including ventricular fibrillation (VF). Typically, ICDs detect tachyarrhythmia by first detecting a rapid heart rate. Other detection methods in addition to fast rate detection are used to reduce the incidence of inappropriate shocks. The present inventors have recognized a need for improved sensing of events related to device recognition of tachyarrhythmia.